Momentum gained in the last decade, including the introduction of mobile vehicular communication systems, is being fully exploited in an international effort to realize the personal communication services (PCS) of tomorrow. In the envisioned PCS, each subscriber carries a pocketsize communication device with an associated personal telephone number. An intelligent global network locates the individual and supervises two-way wireless transmissions which may involve speech, data, fax, and, video streams.
The most important aspect of PCS is wireless communication inside buildings, where people spend most of their time. In a typical wireless indoor application, transmission takes place over a radio link ranging from a few meters to a few tens of meters. Indoor radio propagation, however, is more complicated than transmission between an earth station and a spaceship millions of kilometers away. Signals received inside a building suffer from serious distortions caused by multipath dispersion, and are usually severely attenuated. The channel is dynamic, with its properties changing over space (motion of the portable unit itself) and over time (motion of people and objects around the wireless potable unit). Detailed characterization of the propagation medium is essential in successful design of indoor communication systems.
In a typical indoor portable wireless system, a basestation with a fixed antenna (AP) is installed in an elevated position and communicates with a number of portable/fixed radios (Stations) inside the building. Due to reflection and scattering of radio waves by structures inside a building, the transmitted signal most often reaches the receiver by more than one path, resulting in a phenomenon known as multipath fading. The signal components arriving from indirect paths and the direct path (if it exists) combine and produce a distorted version of the transmitted signal. In narrow-band transmission, the multipath medium causes fluctuations in the received signal envelope and phase. In wide-band pulse transmission, on the other hand, the effect is to produce a series of delayed and attenuated pulses (echoes) for each transmitted pulse. This is illustrated in FIGS. 1A and 1B, wherein the channel's responses at two points in the three-dimensional space are displayed. FIG. 1A presents a point with low delay spread, while FIG. 1B presents a point with a larger delay spread. Both analog and digital transmissions suffer from severe attenuation by the intervening structure. The received signal is further corrupted by other unwanted random effects: noise and co-channel interference.
Multipath fading seriously degrades the performance of communication systems operating inside buildings. Unfortunately, little can be done to eliminate multipath disturbances. However, if the multipath medium is well characterized, the transmitter and receiver can be designed to “match” the channel and to reduce the effect of; these disturbances. Detailed characterization of radio propagation is therefore a major requirement for successful design of indoor communication systems.
Propagation of radio waves inside a building is a highly complicated process. The impulse response approach described here can be used to characterize the channel. A study of the literature shows that the number of multipath components in each impulse response profile, N, is a random variable. Mean value of N is different for different types of buildings. The path variable sequences {ak}, {tk}, θk for every point in space are random sequences. The mean and variance of the distribution of aks are also random variables due to large-scale in homogeneities in the channel over large areas.
Adjacent multipath components of the impulse response profile are dependent. A standard Poisson hypothesis is inadequate to describe the arrival-time sequences. Adjacent amplitudes are likely to have correlated fading for high resolution measurements, since a number of scattering objects that produce them may be the same. Phase components for the same profile, however, are not correlated since at frequencies of interest their relative excess range is much larger than a wavelength. The amplitude sequence and the arrival-time sequence are correlated because later paths of a profile go through multiple reflections and hence experience higher attenuation.
The impulse response profiles for points that are close in space are correlated since the structure of the channel does not change appreciably over very short distances. Spatial correlation governs the amplitudes, the arrival-times and the phases, as well as the mean and variance of the amplitudes. There are small-scale local changes in the channel's statistics and large-scale global variations due to shadowing effects and spatial non-stationarities.
Path loss in an indoor environment is very severe most of the time. It is also very dynamic, changing appreciably over short distances. Simple path loss rules are successful in describing the mobile channel, but not the indoor channel.
The parameters of the channel depend greatly on the shape, size and construction of the building. Variations with frequency are also significant.
In its more general form the channel is non-stationary in time. Temporal variations are due to the motion of people and equipment around both antennas.
Any realistic channel model should consider the above factors. Furthermore, it should derive its parameters from actual field measurements rather than basing them on simplified theory.
A known and a convenient model for characterization of the indoor channel is the discrete-time impulse response (i.e., DTIR) model. In this DTIR model the time axis is divided into minor intervals called “bins”. Each bin is assumed to contain either one multipath component, or no multipath component. The possibility of more than one path in a bin is excluded. A reasonable bin size is the resolution of the specific measurement since two paths arriving within a bin cannot be resolved as distinct paths. According to the DTIR model, each impulse response is described by a sequence of “0”s and “1”s (the path indicator sequence), wherein a “1” indicates the presence of a path in a given bin and a “0” represents the absence of a path in that bin. Each “1”, has an associated amplitude and a phase value.
The advantage of this model is that it greatly simplifies any simulation process. It has been used successfully in the modeling and the simulation of the mobile-radio propagation-channel. Analysis of system performance is also easier with a discrete-time model, as compared to a continuous-time model.
When a single unmodulated carrier (constant envelope) is transmitted in a multipath environment, due to vector addition of the individual multipath components, a rapidly fluctuating CWS envelope is experienced by a receiver in motion. To deduce this narrow-band result from the above wide-band model we let s(t) of (4) be equal to 1. Excluding noise, the resultant CWS envelope R and phase φ for a single point in space are thus given by equation 1:
                              R          ⁢                                          ⁢                      ⅇ            jφ                          =                              ∑                          k              =              0                        ∞                    ⁢                                          ⁢                                    a              k                        ⁢                          ⅇ                              jθ                k                                                                        (        1        )            Sampling the channel's impulse response frequently enough, one should be able to generate the narrow-band CWS fading results for the receiver in motion, using the wide-band impulse response model.
The impulse response approach described in the previous section is supplemented with the geometrical model of FIG. 2. The signal transmitted from the base reaches the portable radio receivers via one or more main waves. These main waves consist of a line-of-sight, i.e., LOS (1) ray and several rays reflected (2) or scattered by main structures such as partitions (3), outer walls, floor (4), ceilings, etc. The LOS wave may be attenuated by the intervening structure to an extent that makes it undetectable. The main waves are randomized upon arrival in the local area of the portable. They break up in the environment of the portable due to scattering by local structure and furniture. The resulting paths for each main wave arrive with very short delays, experience about the same attenuation, but have different phase values due to different path lengths. The individual multipath components are added according to their relative arrival times, amplitudes, and phases, and their random envelope sum is observed by the portable. The number of distinguished paths recorded in a given measurement, and as a given point in space, depends on the shape and structure of the building, and on the resolution of the measurement setup.
The impulse response profiles collected in portable site i and portable site j of FIG. 3 are normally very different due to differences in the intervening (base to portable) structure, and differences in the local environment of the portables. FIG. 4 schematically presents stations/mobiles at different locations compared to the access point (i.e., ‘AP’) wherein some of the stations are mobile and some are stationary.
Variations in the statistics are now described. LetXijk(i=1, 2, . . . , N; j=1, 2, . . . , M; k=1, 2, . . . , L)   (2)be a random variable representing a parameter of the channel at a fixed point in three dimensional space. For example, Xijk may represent amplitude of a multipath component at a fixed delay in the wide-band model, amplitude of a narrow-band fading signal, the number of detectable multipath mean excess delay or delay spread, etc. The index k in Xijk numbers spatially adjacent points in a given portable site of radius 1-2 m. These points are very close (in the order of several centimeters or less). The index j numbers different sites with the same base-portable antenna separations, and the index i numbers groups of sites with different antenna separations.
With the above notations, there are three types of variations in the channel. The degree of these variations depends on the type of environment, distance between samples, and on the specific parameter under consideration. For some parameters, one or more of these variations may be negligible.
It is acknowledged that for small-scale variations, a number of impulse response profiles collected in the same “local area” or site are broadly similar since the channel's structure does not change appreciably over short distances. Therefore, impulse responses in the same site exhibit only variations in details. With fixed i and j, Xijk (k=1, 2, . . . , L) are correlated random variables for close values of k. This is equivalent to the correlated fading experienced in the mobile channel for close sampling distances.
It is further acknowledged that for mid-scale variations, this is a variation in the statistics for local areas with the same antenna separation. As an example, two sets of data collected inside a room and in a hallway, both having the same antenna separation, may exhibit great differences. If μij denotes the mathematical expectation of Xijk (i.e., μij=Ek (Xijk), where Ek denotes expectation with respect to k), then for fixed i, μij is a random variable. For amplitude fading, this type of variation is equivalent to the shadowing effects experienced in the mobile environment. Different indoor sites correspond to intersections of streets, as compared to mid-blocks.
It is lastly acknowledged that for large-scale variations, the channel's structure may change drastically, when the base to portable distance increases, among other reasons due to an increase in the number of intervening obstacles. As an example, for amplitude fading, increasing the antenna separation normally results in an increase in path loss. Using the previous terminology ε(di)=Ejk(Xijk)=Ej(μij) is different for different dis, if Xijk denotes the amplitude, this type of variation is equivalent to the distance dependent path loss experienced in the mobile environment. For the mobile channel ε(d) is proportional to d−n, where d is the base-mobile distance and n is a constant.
A comparison between the indoor and the mobile channels is now provided. The indoor and outdoor channels are similar in their basic features: they both experience multipath dispersions caused by a large number of reflectors and scatters. They can both be described using the same mathematical model. However, there are also major differences, briefly described in this section.
The conventional mobile channel (with an elevated base-station and low-level mobile/fixed station) is stationary in time and non-stationary in space. Temporal stationary is because signal dispersion is mainly caused by large fixed objects (buildings). In comparison, the effect of people and vehicles in motion are negligible. The indoor channel, on the other hand, is not stationary in space or in time. Temporal variations in the statistics of the indoor channel are due to the motion of people and equipment around the low-level portable antennas.
The indoor channel is characterized by higher path losses and sharper changes in the mean signal level, as compared to the mobile outdoor channel. Furthermore, applicability of a simple negative-exponent distance-dependent path loss model well established for the mobile channel is not universally accepted for the indoor channel.
Rapid motions and high velocities typical of the mobile users are absent in the indoor environment. The Doppler shift of the indoor channel is therefore negligible.
Maximum excess delay for the mobile channel is typically several microseconds if only the local environment of the mobile is considered, and more than 100 μs if reflection from distant objects such as hills, mountains, and city skylines is taken into account. The outdoor rms delay spreads are of the order of several μs without distant reflectors, and 10 to 20 μs with distant reflectors. The indoor channel, on the other hand, is characterized by excess delays of less than one μs and rms delay spreads in the range of several tens to several hundreds of nanoseconds (most often less than 100 ns). Therefore, for the same level of inter-symbol interference, transmission rates can be much higher in the indoor environments.
Finally, the relatively large outdoors-mobile transceivers are powered by the battery of the vehicle with an antenna located away from the mobile user. This is in contrast with lightweight portables normally operated close to the user's body. Therefore, much higher transmitted powers are feasible in the outdoors-mobile environment.